Myelomas in mice and humans are detectable when the immunoglobulin class produced by the tumor is elevated or when the tumor is manifested in vivo. The MOPC 104E murine myeloma is unique in that it produces a tumor specific circulating marker (IgM (gamma) that permits tumor detection even in the presence of normal levels of immunoglobulins and allows quantitation of normally undetectable tumor clones. MOPC 104E myeloma specific IgM (gamma) reacts with dextran B-1355. Low but measurable amounts of tumor specific IgM (gamma) anti-dextran continue to circulate in animals apparently cured by chemotherapy. The cells in this state are defined as representing a "clinically stable tumor clone" (CSTC). Since IgM (gamma) anti-dextran is a specific marker produced only by the original tumor, we shall use it a) to determine if CSTCs remain stable or relapse as the animals age, b) to establish the mechanisms of control in vivo as to whether the immune system or another mode of tumor control is operating and c) to test the concept that the destruction of tumor cell number to a critical plateau level is sufficient to contain the remaining neoplastic cells. Understanding the mechanism operating in the host which allows each individual to naturally control his own myeloma clones would be a significant advance over the current methods now available.